Unravelling ‘S’ and ‘D’ values.

Most towing equipment introduced since 1998 should be type approved and have a plate or sticker showing:

• The Manufacturer

• The country of Type Approval (eg E11)

• The part number

• The Eu standard (typically A50-X)

• The ‘S’ Value

• the ‘D’ Value.

As users, the 2 most important bits of information are the ‘S’ value and the ‘D’ value.

The ‘S’ value is straightforward. Simply, it is the maximum vertical load (nose-weight) that can be imposed on it. It’s measured and shown in kilograms and typically might be between 50 -150 Kgs. Higher values are permissible on certain equipment but, for the purposes of this blog, it’s restricted to regular use such as a Motorhome owner or a car/van driver towing his/her Cat01 or Cat02 trailer on a standard 50mm tow-ball. (Trailer category info is for another blog).

The ‘D’ value is a little more complex. Towing equipment isn’t generally rated by a finite value (i.e. 1500kgs) but by a ‘D’ value calculation. The capacity of the towing equipment will vary, dictated by the GVW (Gross Vehicle Weight) or MAM (Maximum Authorised Mass) of the towing vehicle. As the vehicle’s GVW or MAM goes up, it’s towing capacity will go down if the ‘D’ value is fixed.

‘D’ value of the towing equipment needs to be at least high enough to accommodate the vehicle’s maximum towing capacity as calculated from the VIN plate.

Here we have the VIN plate from a Vauxhall Vivaro Van.

The info on the VIN plate shows a towing limit of

exactly 2 tonnes (fully laden) . That is the Gross

Vehicle weight (1st weight displayed) deducted

from the Gross Train Weight (2nd weight

displayed).

To be compliant, the ‘D’ value of the towing

equipment needs to accommodate both the van’s

GVW (MAM) and it’s maximum towing limit.

So How Do We check it’s Adequate?

Use the calculation:

D value = 9.81 x (towing vehicle’s GVW x Trailer’s MAM) / (towing vehicle GVW + Trailer’s MAM)

In this case, 9.81 x(3.03(tonnes)

x 2(tonnes)) / (3.03(tonnes) + 2(tonnes)) = 11.82 D value.

As the tow-bar is plated at a D value of 12.02, it’s a

perfect match! :)

NOTE! It is ILLEGAL to exceed ANY of the weights on the VIN plate, or the towing equipment’s nose-weight (in this case 85Kgs.)

WEIGHTS & VIN PLATES!

The information here is based on a motorhome conversion of a commercial chassis but it holds true for virtually any vehicle.

VIN stands for Vehicle Identification Number.

It's a unique 17 digit number which includes coded information such as Country of Assembly, the Manufacturer, the Body Style and Engine, and the year of build. The last 6 digits are the vehicle's serial number and when all are combined, it's the unique 'fingerprint'. It should be stamped in the chassis but may also be found around the engine block, the driver's door jamb and probably in the boot.

Additionally, it should be stamped on a VIN plate (usually located just under the

bonnet) and the plate will also show various weights.

When a vehicle has been converted or modified (i.e. campervans/motorhomes) the converter will add there own plate or sticker which may modify the weights.

It's vital you understand your vehicles VIN plate. Exceeding ANY ONE of the listed weights is an offence.

What weights are shown? From top to bottom in the example shown is the MAM (Maximum Authorised Mass), GVW (Gross Vehicle Weight) or MGW (Maximum Gross Weight) followed by GTW (Gross Train Weight) or GCW (Gross Combined Weight).

The numbers 1. 2. and 3. are axle weights. 1. is the maximum allowable weight of the front axle. 2. is the maximum allowable weight of a single rear axle. 3. is the maximum allowable weight of a third or 'tag' axle.

Axle weights added together should exceed the GVW which allows for some margin in the way the vehicle is loaded.

What weights are NOT shown? The VIN plate will not show the unladen or kerb weight, the payload, the towing limit or the towbar noseweight.

The unladen or kerb weight should be in the vehicle handbook. However, to be sure, have the vehicle weighed on a weighbridge. The difference between the unladen weight and the MGW is the maximum allowable PAYLOAD.

The towing limit can be calculated by deducting the MGW from the GTW. The resulting figure is assuming that the vehicle is fully laden. In the event that the full payload isn't being used, the balance can be added to the calculated towing limit.

The towbar itself should have a type approval sticker which shows a 'D' value and an 'S' value. The 'S' value quite simply is the maximum allowable noseweight. The 'D' value is rather more complicated and is a calculation based on the GVW and the maimum towing limit. I shall cover this in another blog.

REMEMBER! 1. Do NOT exceed any of the weights on the VIN plate.

2. Check your driving licence categories and limits to ensure you are legal!

Check Noseweight for Stability!

Whatever trailer you tow, stability is all important.

Snaking is not an uncommon problem on high sided trailers such as caravans, or on combinations with a long overhang on the trailer. There are aids that can be used to help control it but getting the noseweight right initially is fundamemtal.

Commercial noseweight gauges are available such as the MP970 Noseweight Indicator Gauge from Maypole. https://www.maypoleltd.com/product/mp970-noseweight-indicator-gauge/.

However, bathroom scales, used correctly, will work equally well!

Using my method, checking the weight of both single and double (or multi) axle trailers is straightforward.

Towball height is critical! Especially on twin or triple axle trailers. On multi axles, If you need to push the coupling downwards on to the towball, the noseweight is NEGATIVE and the unit will be highly unstable. Having to push downwards puts more weight on the trailers front axle and in extreme circumstances it can be overloaded while the rear axle(s) are doing almost nothing. If you need to lift the coupling up to get it onto the ball, noseweight could be excessive and over the allowable limits of the towball, tow-bar and vehicle chassis. The trailer's front axle will be unloaded while the rear is over loaded.

STEP 1. On level ground, locate the trailer onto the towball and lift the jockey wheel clear, if fitted. Towing vehicles vary but with soft suspension, it's possible the ball height could be significantly lower when laden compared with unladen.

STEP 2. Measure the height from the ground to the centre of the towball/coupling head. Record this height when the towball is laden.

STEP 3. Unhitch the trailer and replace the towball with the noseweight guage, set at the prerecorded height.

If using bathroom scales, ensure they are on a flat surface, protect the platen with a sheet of plywood or similar and use a broom handle cut to the required length, allowing for the depth of the scales. Record the noseweight.

STEP 4. If the load needs to be adjusted, REMOVE the noseweight guage, ensure the jockey wheel is down and the handbrake is securely applied. With a car on a transporter, just a few inches backwards or forwards will make a big difference to the noseweight. The ideal weight is likely to be NOT LESS than 50kg, aiming for around 7% of the gross trailer weight. (1 tonne gross = 70kgs noseweight).

STEP 5. With a car on a transporter, once the desired weight is achieved, be sure to fix chocks to the deck, in front of the front wheels. On subsequent uses, you should then be able to just drive straight up to them and be right first time. ENSURE the vehicle is securely strapped down!

DO NOT exceed the maximum allowable noseweight of the vehicle chassis (should be shown in the handbook) or of the coupling, the towball or the towbar. Noseweight on type approval plates is shown as the 'S' value in kgs.